The use of plastic or coated paper transfer sheets is well known. An image is first printed on such a sheet and then, generally using a heat/pressure transfer process, the image is transferred to a final substrate, which may be a shirt, cup or other object. The image is fixed to the transfer sheet so that the sheet may be stored, handled, etc., without damage to the image. Often, the sheet is first coated with an interlayer to which the ink is attracted so that good printing onto the sheet is achieved. Generally, the bonding of the interlayer to the transfer sheet is purposely made poor enough so that during transfer to the final substrate, the ink, together with the underlayer, are transferred to the final substrate. Alternatively, the bond between the ink and the underlayer is weak enough so that the ink separates from the underlayer and transfers separately to the final substrate.
This process is to be distinguished from offset printing in which an intermediate transfer member is also used. In offset printing processes, the ink is not fixed to the intermediate member. In standard ink offset printing the ink layer on the intermediate member splits during transfer to a substrate, so that about half of the ink on the member transfers to the substrate to form the image. In toner offset processes, the image on the intermediate member is substantially completely transferred to the substrate, usually by heat and pressure. Alternatively or additionally, in some systems electrostatic fields are used to aid or effect transfer of the toner image.
The process of offset printing, and especially of toner offset printing, on transfer sheets has not been fully solved. In particular, when using liquid toner to print on a sheet, three different situations are known in the art:                1) To some sheet materials, such as paper and Teslin (polyethylene mixed with silica) transfer is good. However, fixing to those materials is usually so good that image transfer to a third substrate is usually unsuccessful.        2) Transfer to many materials is per se very difficult. For example, printing with liquid toner in which the toner particles are based on a copolymer of ethylene with methacrylic acid on most plastics such as bare polyester or polypropylene is difficult or impossible.        3) Treatment of plastics to improve spring of liquid toners thereon is known. However, such treatments, be they in the nature of surface treatments or coatings generally result in good fixing of the image to the substrate, such that transfer from the sheet is generally unsuccessful.        
It should be understood that the processes described above are seldom perfect and, for example, there are many cases in which some of the ink or toner will transfer from the sheet to a final substrate. However, the image will not completely transfer or will tear and some of the image will remain on the sheet.
An exemplary system for permanently printing at least liquid toners on plastic sheets is described in WO 99/19773, the disclosure of which is incorporated herein by reference. This reference teaches, inter alia, the coating of a plastic sheet with two layers. The layer closest to the sheet is tightly bonded to the plastic sheet, but generally has a low affinity and/or bonding strength with the toner. An upper layer, bonds well to both the lower layer and the toner.